Idaho murders: BTK serial killer on solitary confinement and Bryan Kohberger, 'I know how he feels'

EXCLUSIVE: The BTK serial killer says he can relate to the loneliness Bryan Kohberger is staring at behind bars — while confirming he never spoke with the criminology student who studied under his true crime co-author.

Dennis Rader — the "Bind, Torture, Kill" murderer — spent months in custody awaiting court proceedings, just as the Idaho murders suspect is doing now.

"Since I spent from February 2005 to April 2005 in a cell by myself I know how he feels," he told Fox News Digital. "Very lonely."

Rader met with his defense attorney every day, he added, and found ways to pass the time.

IDAHO MURDERS SUSPECT BRYAN KOHBERGER STUDIED UNDER EXPERT ON SERIAL KILLER BTK; DAUGHTER ‘SICK’ AT NEWS

During frequent meetings with his public defenders, Rader wanted to go through "everything" prosecutors had on him, even though he had confessed on the day of his arrest, according to his daughter, Kerri Rawson.

"He was going back and forth between pleading not guilty and guilty," she told Fox News Digital. "One difference is dad confessed and seemed relieved, versus [Kohberger] has to feel very alone."

Kohberger, according to prosecutors in Pennsylvania, may have waived extradition out of a "need to know" how investigators established probable cause after the crime.

Once in custody, Rader began receiving letters and artwork from strangers.

"And soon letters started to come in," he said. "[I] read a lot of the Bible and wrote poetry."

His writings during that time include an infamous missive titled "Black Friday," seemingly about his arrest and signed, "The Suspect."

"Dad had fan mail very early, and I’m seeing it being sent to Bryan," Rawson said. "And art being made — like women [are] in love with these guys."

Kohberger, 28, is being held without bail at the Latah County Jail in Moscow, Idaho — the same town where he allegedly snuck into a six-bedroom home in the middle of the night and ambushed four young people, some of whom may have been sleeping at the start of the attack.

IDAHO MURDERS: XANA KERNODLE'S MOTHER SAYS LAWYER ‘BETRAYED’ HER TO DEFEND BRYAN KOHBERGER

Police have identified the victims as 21-year-olds Madison Mogen and Kaylee Goncalves and 20-year-olds Xana Kernodle and Ethan Chapin. 

All but Chapin were housemates, and all four participated in campus Greek Life. Chapin, who lived in the Sigma Chi frat house nearby, was visiting for the night and was dating Kernodle. Two other young women inside were not attacked, including one who told police she saw a masked man leaving through a sliding door.

Kohberger, who is reportedly a recovering heroin addict with socially awkward tendencies, studied for his master's degree in criminal justice at DeSales University under Dr. Katherine Ramsland, an expert on serial killers who co-authored a book with Rader.

Rader, like Kohberger, had a degree in criminal justice.

IDAHO MURDER: VETERAN DEFENSE ATTORNEY LAYS OUT CASE BRYAN KOHBERGER TEAM COULD MAKE, ‘HOLES’ IN AFFIDAVIT

Although criminology students commonly send messages to Rader, the inmate told Fox News Digital that the Idaho stabbings suspect never tried to reach out to him.

After attending DeSales, Kohberger went on to seek a Ph.D. in the department of criminal justice and criminology at Washington State University, just a few miles from the home where police allege he brutally stabbed four University of Idaho undergrads on Nov. 13.

WSU has a specialized "Complex Social Interaction Lab" that includes a database of police bodycam footage from more than five departments.

A WSU spokesman last month told Fox News Digital that Kohberger was never granted access, and Moscow police said they don't share video with the program.

But Kohberger did apply for an internship with local police, investigators allege — and according to documents obtained by the New York Times, he tried to gain access to the CSI Lab directly by applying for a graduate research assistantship position.

BRYAN KOHBERGER CASE: THEORY IDAHO SUSPECT WAS BEHIND PARTY HOUSE NOISE COMPLAINTS DEBUNKED

Although Kohberger allegedly stalked the King Road victims' home a dozen times before the slayings, it does not appear that he had a connection to three noise complaints there over the past semester — or the resulting bodycam videos.

WATCH: Moscow police bodycam shows Kaylee Goncalves speak with officers during noise complaint response 

Two of the three complainants, on the evening of Sept. 1 and the early morning of Sept. 2, are identified in police reports as neighbors of the King Road home in Moscow. Both calls came from houses on Walenta Drive, the street behind the six-bedroom off-campus rental house.

An earlier incident report from Aug. 16 does not include a complainant's name. However, the responding officer, on bodycam video, repeatedly mentions that the loud music is disturbing neighbors. 

WATCH: Moscow police bodycam shows Xana Kernodle speak with officers during noise complaint response

After the slayings, Kohberger finished the semester at WSU both in his Ph.D. program and as a teaching assistant.

Pennsylvania police and the FBI arrested him on Dec. 30 at his parents' house in the Poconos, more than 2,500 miles away from the crime scene. He waived extradition and made his first appearance before an Idaho judge on Jan. 5.

Kohberger is due back in court on June 26 for the start of his preliminary hearing, which could take four to five days.

CLICK HERE TO GET THE FOX NEWS APP

In a rare public statement speaking through his former Pennsylvania attorney, Jason LaBar, Kohberger has said he's "looking forward to being exonerated."

Kohberger's Idaho attorney, Kootenai County Public Defender Anne Taylor, plans to challenge the probable cause for his arrest during the June proceedings.